This invention relates to sensing articles at a delivery station in a vendor and more particularly to a multiple-beam optical sensing system for sensing such articles.
It has been found to be advantageous in present vendors to have some means for sensing if an article is present at the delivery station, i.e., at the place where the customer physically removes the vended article from the vendor. For example, such sensing is desirable to prevent a second article from being vended when a first article is still present at the delivery station.
Present sensor systems, using multiple beams, exemplified by U.S. Pat. No. 4,108,333, perform this sensing function well. But there is some room for improvement. For example, the sensor system of the abovementioned patent has two emitters and two detectors, the emitters being disposed on the left-hand side of the delivery station and the detectors being disposed on the right-hand side of said station. Both emitters are "on", i.e., emitting electromagnetic radiation, at the same time. The power consumption while the emitters are on is, of course, approximately double that of a single emitter. It is necessary to use two emitters to ensure that no article at the delivery station remains undetected. The emitters of the abovementioned patent can be pulsed, which reduces power consumption, but this pulsing generates a considerable amount of noise in the system, which must be filtered out.
Because the emitters of the abovementioned patent are both on the same side of the delivery station, there exists the possibility that cross talk could be a problem. If one of the emitters does not emit a sufficiently focused beam of electromagnetic radiation, that radiation might be detected not only by its detector, but also by the second detector. Of course, if an article at the delivery station were relatively close to the second emitter so as not to fall within the unfocused beam of the first emitter, it would not be detected. The first emitter's beam would be detected by both detectors thereby indicating the absence of an article at the delivery station even though the second emitter's beam would be blocked.
The potential problem of cross talk can be eliminated by having one emitter and one detector on each side of the delivery station. This arrangement also has the desirable result that identical units, i.e., units consisting of one emitter and one detector, can be used on both sides of the delivery station. There are problems with this arrangement too, however. Such a system, for example, behaves like a reflective system if the article present at the delivery station is sufficiently reflective to the electromagnetic radiation of the emitters. In such circumstances, rays from an emitter on one side of the delivery station are likely to be reflected back to the same side and detected by the detector on that side. If this also happens with the emitter-detector pair on the opposite side of the delivery station, no article is detected since the detectors have no way of telling from which side the rays originated.
Another problem arises in present systems when an article only partially interrupts the beam from one emitter and does not interrupt the beam from the other emitter. All systems have thresholds of detection, and if this article is right on the threshold of bring detected, ambient light sources (such as electric light bulbs whose output oscillates at 60Hz) can cause the system to oscillate between detecting and not detecting the article. This "chatter", especially if rapid, is highly undesirable.